Li-Ion battery packs that consist of more than one Li-Ion battery cells connected in series are commonly used in applications such as notebook PCs, cordless power tools, electric vehicles, uninterruptable power supplies, and so forth. An important function of the battery management circuitry for such packs—typically an Integrated Circuit (IC), is to manage cell balancing, which is important to the overall battery pack capacity and runtime. Cell balancing is the process of matching the voltage across each individual cell in the battery pack. Unbalanced cells can lead to premature charge termination, early discharge termination, or further cell abuse from cycling above or below optimal cell voltage limits.
Many factors can cause cell imbalance, such as cell-to-cell capacity mismatch, state-of-charge difference, cell impedance variation, temperature gradients, and cell self-heating at high discharge rates. Various methods and algorithms have been developed to balance cells in multi-cell battery packs for optimal performance. For example, passive (“bleed”) balancing shunts energy around a cell by converting it to heat in a bypass power resistor and a switch, typically a Field Effect Transistor (FET), while charge shuffling methods capacitively redistribute energy between the cells.
Each cell balancing method has its own advantages and disadvantages. Passive balancing is usually considered as the least expensive approach because it does not require a discrete capacitor or inductor. The balancing control algorithm is relatively simple although the excess energy is essentially wasted as heat, and in many cases, it is still the method of choice due to its low cost in both hardware and software/firmware. In other cell balancing schemes, external transistor networks may be employed to balance voltages across the cells, however, such external networks require additional integrated circuit control pins to operate the transistors which can lead to increased circuit costs.